1. Field of the Invention
The present invention relates to an input circuit for sampling an analog signal. In particular, the present invention relates to an input circuit for sampling an analog differential signal with a wide common mode range.
2. Discussion of the Related Art
In an analog-to-digital converter (ADC), it is higher desirable to have an input circuit that allows sampling a fully differential signal. It is particularly desirable when the input circuit can sample a differential analog signal with a magnitude that is greater than a diode drop, and performs that sampling over a large common mode range.
FIG. 1 shows a switching circuit that is disclosed in U.S. Pat. No. 8,022,679 (“the '679 patent”), entitled “SYSTEMS AND METHODS FOR FAST SWITCH TURN ON APPROXIMATING IDEAL DIODE FUNCTION.” As shown in FIG. 1, the switching circuit of the '679 patent is not fully capacitively isolated, and its charge pump is not referenced to a source terminal of the switch. Also, the switching circuit has neither a discriminator nor a voltage regulator to provide a well-controlled gate-to-source voltage (VGS). Therefore, the switching circuit of the '679 patent cannot operate below the ground reference and thus is not suitable for use in a sampling network for an ADC.
FIG. 2 shows a bootstrapped switch circuit disclosed in U.S. Pat. No. 7,176,742 (“the '742 patent”), entitled “BOOTSTRAPPED SWITCH WITH AN INPUT DYNAMIC RANGE GREATER THAN SUPPLY VOLTAGE.” The bootstrapped switch circuit of the '742 patent may be used for sampling an input signal that goes beyond the power supply voltages. However, this bootstrapped switch circuit is capable of accepting an input signal that exceeds the supply voltage on the positive side by a limited amount, as the several low voltage transistors in the bootstrapping circuit is unable to withstand higher voltage signals. As shown in FIG. 2, the bootstrapped switch circuit of the '742 patent includes capacitor C13 that is precharged by being first connected between supply voltages GND and VDD. The charged capacitor is then switched to being connected between the source and gate terminals of bootstrapped switch N20. For high input signals, precharged capacitor C13 must swing up to the input signal. The parasitic capacitors in bootstrapped switch MMN20 result in a signal-dependent VGS.
FIG. 3 shows a bipolar switch that is disclosed in U.S. Patent Application Publication 2013/009623 (“the '623 publication”), entitled “FOUR-QUADRANT BOOTSTRAPPED SWITCH CIRCUIT.” The bipolar switch in the '623 publication is operated by a floating voltage source. The switch of the '623 publication cannot accept an input signal that can go beyond the supply voltages, as the floating voltage source is generated by current sources that operate between those supply voltages.